The efficiency of a conventional switched power converter, such as a buck converter, is dominated by the losses within the switches (e.g. field effect transistors, FETs) and the inductor of the power converter. If the power converter is supplied from a relatively high input voltage Vin, the power converter typically exhibits a reduced conversion efficiency, because the switches must be implemented in high voltage technology, and thus the switches have an increased switch area and increased reverse recovery losses. Relatively large FETs typically cause relatively high switching losses, because of an increased gate charge and LX capacitance.
The voltage which is applied to the inductor of a buck converter is proportional to the difference between the input voltage Vin and the output voltage Vout, i.e. Vin−Vout, during the magnetization phase, or proportional to −Vout during the demagnetization phase. Increased inductor voltages cause increased current variations dI/dt and thus an increased switching frequency (for achieving a pre-determined current ripple) and/or an increased current ripple (for a given switching frequency). In both cases this leads to increased inductor core losses and to an increased dissipation power.
Maintaining low current variations dIL/dt at increased input and output voltages typically requires inductors (i.e. coils) with increased inductance L, due to the relation dIL/dt=VL/L. However coils with an increased inductance L have an increased number of turns. For inductors to maintain their Direct Current Resistance (DCR) even with an increased number of turns, each turn has to make use of a wire with an increased thickness to compensate for the impedance increase. Thus the size of the inductor is growing twice with an increased inductance L (due to the increased number of turns and due to the increased wire thickness). On the other hand, if the inductor dimensions are not increased, an increased inductance L leads to the effect that the DCR of the inductor is growing twice due to the additional number of turns and due to the use of a thinner wire.
Over the last years battery powered applications (like smartphones and tablets) increased their computing power, screen resolution and display frame rate and added connected standby modes. This caused an increased drain of the battery of such devices, so that electronic devices such as smartphones typically need to be re-charged on a daily basis. The limited mobility time of battery powered electronic devices may be addressed by using battery packs with an increased capacity, but a re-charge of such battery packs requires relatively long time intervals. This is caused by the fact that most of the electronic devices are charged through a standard (Micro) USB port, which provides limited current handling capability (˜1.5 A). Therefore a 5 Ah battery pack requires multiple hours for re-charging, even if the battery technology (typically Lilon/LiPolymer) would allow a re-charge within less than one hour (1-2 C charging).
Recent changes in the USB charging specification allow voltages higher than the standard 5V, enabling more than four times the power from the USB supply (9V, 12V and 20V). However, due to the fact that electronic devices are space and height constrained (especially regarding the inductors used for switched power converters), an increased input voltage Vin of a power converter (provided e.g. via the USB port) cannot be compensated by using inductors of higher inductance L. As a result of this, either the DCR of the inductor is increased or the switching frequency has to be increased. Both measures lead to an increased dissipation power and possibly hot spots at the housing of an electronic device.